So far, we request a certificate each time we start the broker.  Let's
Encrypt maintains several rate limiters and if we exceed one of them, we
won't get a certificate.  Worse, since we don't store certificates, we
won't even be able to use an old one.

This patch uses autocert's DirCache structure to cache certificates on
disk.

This patch fixes <https://bugs.torproject.org/30512>.

MaxBytesReader is only documented for server side reads, so we're using
a local limitedRead function instead that uses an io.LimitedReader.

Declared limits in a commented constant

This is a fix for #26348

Amounts to the same thing as currently implemented in go-webrtc

Similar to c28c8ca4 for proxy-goc

NewBrokerContext now requires a logger so give it one to discard the
logs.

Now that the page is no longer necessary.

Otherwise, this is set on the prototype and shared across instances :/

Note the change to the test that highlights this bug.  A proxy pair was
already created when an earlier test called `beginWebRTC()` but a new
Snowflake instance should start with an empty array.

Previously the metrics log file was hardcoded and the broker wasn't
behaving properly if it was unable to open the file for logging.

Added a commandline option to specify the logfile that defaults to
Stdout.

Fixed up some documentation and log output formatting

For now we can just log the proxy geoip statistics locally

This fixes a bug introduced by the deadlock patch for ticket #25688.

Our log scrubber was a bit over-zealous and was scrubbing timestamps as
well. While we're still over-scrubbing, we're now more precise and
ensure that compressed IPv6 addresses have "::"

The browser icon shows the on/off state.

Permits further ui extension

From https://trac.torproject.org/projects/tor/ticket/23888#comment:5

And inline its only use site.